The operation of large ocean going vessels requires an enormous amount of energy in the form of heavy oil. On one hand, this has ecological implications such as the emission of greenhouse gases; on the other hand bunker cost is by far the biggest monetary factor when operating vessels. Our ideas target the optimal maintenance of the vessel hull and propeller in order to reduce fuel costs and pollutant emissions. Every time a vessel is under anchor, organisms will attach to its body and propeller. Studies have shown that poor hull and propeller condition will lead up to 30% more fuel consumption compared to a clean state. With our research, we first aim for estimating the additional fuel consumption due to the unobservable hull and propeller fouling from longitudinal operational data that are collected from various sources (fuel consumption on board, GPS position data, weather and oceanographic data, et cetera). At the same time, the effect of maintenance actions such as propeller polishing, hull cleaning or dry docking the vessel are pinned down with statistical methods. Finally, optimization methods are used for finding an optimal maintenance plan for any given vessel, including future fuel, maintenance and opportunity costs.